Method of making compact high density radiation screening material containing tungsten



METHOD OF MAKTNG COMPACT HIGH DENSITY RADIATION SCREENING MATERIALCONTAIN- ING TUNGSTEN Jacob Kurtz, 736 Grange Road, Teaneck, NJ.

N Drawing. Filed Nov. 12, 1958, Ser. No. 773,174

1 Claim. (Cl. 75-212) The present invention relates in general to a newand useful composition of matter having the characteristics of a highdensity and of absorbing or screening radiations of X-rays and gammarays without producing excessive secondary gamma ray radiation uponimpingement of such rays upon it Although the present application alsodescribes a product comprising a compact high density radiationscreening material containing tungsten, product claims directed to suchmaterial are the subject matter of an application filed on June 18,1959, being a division hereof.

The shielding materials of the prior art, although efficient as shields,are in many instances of little practical or commercial use because theydo not have the structural strength to be self-supporting. Therefore,when it is desired to shield an area from radiation of the typedescribed it is necessary to construct a composite element a portion ofwhich has the shielding element and another portion of which has thestructural supporting element. It is obvious that a material which hasthe ability to serve as both the shielding element and the structuralsupporting element offers many practical and commercial advantages.

It is an object of the present invention to describe and claim a methodof making such a useful composition of matter which has a combination ofthese advantages, radiation shielding and structural strength.

In the past lead used alone and certain alloys of tungsten with nickeland copper have been used as radiation shields with varying degrees ofsuccess. Lead alone is easily deformable, is mechanically weak andrequires special handling. Tungsten and its known alloys are expensiveand are very difficult to work with because of their hardness whichmakes them difficult to machine and otherwise fabricate to desired sizesand shapes.

1 have discovered that the combination of lead and tungsten combined inthe special manner and by the special method hereinafter describedproduces a new and commercially useful product which has the qualitiesdesired, radiation shielding and structural strength.

Attempts have been made in the past to combine lead and tungsten bymeans such as melting the lead and mixing powdered tungsten therein.This resulted in segregation of the two metals upon solidification ofthe lead without an alloy or even alloy-like structure being formed. Nouseful composition of matter resulted.

The present application discloses and claims an improved method ofmaking such is a new and useful composition of matter including acombination of tungsten and lead in a compact body of high densitycapable of absorbing or screening radiations and describes preferredmethods of manufacture of said composition of matter as brought out andexemplified in the disclosure hereinafter set forth, the scope of theinvention being indicated in the appended claim.

While a preferred embodiment of the invention is described herein, it iscontemplated that considerable variation may be made in the method ofprocedure and the 2,986,465 Patented May 30, 1961 combination ofelements, without departing from the spirit of the invention.

I prepare my novel and useful composition of matter by using thematerials and following the procedures hereinafter set forth. First,granulated tungsten powder particles are prepared in the usual manner tohave a bulk density in the order of 60-140 grams per cubic inch. Thenthese prepared particles are treated by placing them in an aqueoussolution containing decomposable salts of lead, tin, copper and nickel.The solution is then evaporated and upon evaporation particles of all ofthe metal salts present are deposited as a coating on the tungstenparticles.

These coated particles after being sieved through a screen of suitablemesh, say 200, are placed in a suitable container and heated todecompose and reduce the metal salts. In order to accomplish the desireddecomposition and reduction of the salts the heating should be done in areducing atmosphere. The reducing atmosphere may be provided in any wellknown and conventional way such as by heating in a furnace chamber thatis completely evacuated or contains an atmosphere of hydrogen or of asuitable inert gas. Another alternative is to heat the particles in agraphite boat. The formation of this metallic coating is beneficial tothe promotion of a honeycomb type of structure which, as is laterexplained, is an essential step in the formation of the compact bodymade of my novel composition of matter.

The next step is to compress the tungsten particles now covered with ametal coating of alloy-like nature in a suitable mold under controlledconditions of pressure and temperature to produce a pressed metallicbody having a honeycomb structure. The amount of pressure applied andthe temperature used are regulated to produce a pressed body having apredetermined desired density. The next step, which is accomplishedwhile the body is still in the mold is to add lead in the form of shotor in any other suitable and convenient form and to heat the mixture ina suitable furnace or by means of high frequency coils placed over themold to a temperature above the melting point of lead to cause the leadto become molten and run into the voids in the pressed body whereby saidvoids therein are filled and the final product is formed. This can thenbe ejected from the mold. The heating should be done in a protectiveatmosphere to prevent oxidation. This protective atmosphere can beproduced by placing the mold and lead in a chamber that is completelyevacuated or contains hydrogen or a suitable inert gas.

An alternate method is to place the coated tungsten particles, after themetal salts forming the coating thereon have been decomposed andreduced, between a series of heated rollers and to pass themtherethrough and thus to compress the powder to a predetermined desireddensity. When this density is reached sheets of lead are placed on topand bottom of the resulting pressed tungsten body and between that bodyand a series of rollers. Further compression between and heating by therollers will force the lead through the interstices in the surface ofthe pressed tungsten body and into the voids therein. The compressionand heating by the rollers is continued at a temperature just below themelting point of lead until all voids are substantially filled with leadand a homogenized compact of full desired density is obtained.

I In some instances it has been found advantageous to reinforce thecoating on the tungsten particles. A preferred way of doing this is toimmerse the coated particles before they have been heated and pressedinto a pressed tungsten body in an aqueous solution of lead nitrate. Thesolution is then evaporated whereby lead nitrate is added to the coatingon the tungsten particles and this is then decomposed and reduced in themanner described above 3 whereby metallic lead becomes an integral partof the alloy-like metal coating on the particles. These particles arethen sieved through a screen of the size desired and are pressed into abody having a honeycomb structure as has been previously described. Leadis then added by "either of the methods described above to fill thevoids therein. It has been found under some conditions that this methodresults in a higher content of lead in the final compact and, therefore,in a compact of higher density.

The final density of bodies made as above described is determined by thedensity of the pressed tungsten body before any of the lead is added andby the percentage of voids still remaining in the body after theaddition of the lead. The latter is controlled by the amount of leadadded, the temperature at which it is added and the time allowed for thelead to enter the voids. Densities in excess of 99% of the theoreticalmaximum density have been attained.

The composition of matter formed by the above described novel treatmentof tungsten particles with lead comprises a compact body having strongbonds between the particles of coated tungsten and the lead filling thevoids of the original honeycomb structure. Due to the production of analloy-like coating on the tungsten particles the resultant pressedtungsten body develops a strong catalytic-like action in making thecomposite body one of high strength and high density by promoting thesubstantially complete filling of the voids. The densities of bodiesproduced in this manner range from 12.0-18.5 grams per cubic centimeter.The compacts so produced are structurally strong, readily machineableand are not deformable even at elevated temperatures. Specimens havebeen subjected to standard tensile tests and have shown tensilestrengths of from 12,000 pounds per square inch to 25,000 pounds persquare inch, the diiference depending upon the pressure used to form thepressed tungsten body before the lead was added. Standard compress'iontests showed specimens to have a value of from 20,00030,000 pounds persquare inch. From these values it is obvious that the composition ofmatter invented by me which forms the compact has tensile andcompression strengths well within the ranges making them suitable asself-supporting structural elements.

The following are specific examples of compact bodies of the novelcomposition of matter produced by preferred embodiments of my novelmethod and are given by way of illustration, but do not in any way limitthe scope of this invention.

Example 1 One kilogram of pure tungsten powder having a bulk density ofsubstantially 60 grams per cubic inch was first treated with an aqueoussolution containing stannic chloride, nickel nitrate and cupric nitrate.The amounts of the metal salts present in the batch of solution intowhich the particles were immersed were computed to have a combined metalcontent of 5 grams or 0.5% of the weight of the tungsten particlesimmersed in the solution. This comprised 1.25 grams of tin, 2.5 grams ofnickel and 1.25 of copper, thus, the ratio of the metal contents of thespecific salts used was 50% by weight of nickel and 25% by Weight eachof copper and tin. After immersion and thorough mixing in the solutionheat was applied to evaporate the solution. Constant stirring duringheating was necessary to assure uniform distribution of the metal saltson the surface of the tungsten particles to form a coating. Thecoatedparticlesthen were sieved through a 200 mesh screen and thescreened particles were then placed in a nickel boat and heated in areducing atmosphere whereby the particles of metal salts on the surfaceof the tungsten particles were decomposed with the result that thecoating was converted to an alloy-like coating of tin, nickel andcopper.

500 grams of these coated particles were then placed in a mold and againheated and compressed to a density of 10.4 grams per cubic centimeter(170 grams per cubic inch). When this density was reached there wasadded in the mold 260 grams of lead in the form of shot around thecompressed particles now forming a pressed body. This amount wasslightly in excess of the theoretical amount required to fill the voidsin the pressed body. The mixture in the mold was then heated in afurnace chamber containing an atmosphere of hydrogen to a temperature of850 C. for a period of approximately 15 minutes during which period allthe lead had become molten and had seeped down through the honeycombstructure of the compacted body of coated tungsten particles. The moldwas allowed to cool and the compact ejected therefrom. Excess lead wasremoved and the density of the piece was determined by waterdisplacement. For the density to which the tungsten body was pressedbefore addition of the lead in this example, the theoretical densitywould be 15.6 grams per cubic centimeter if all the voids were filled.The actual density of the compact made as described in this example,measured by water displacement, was 15.4 grams per cubic centimeter.

Example II The composition of matter produced by this example was madein substantially the same manner as that described in Example I exceptthat before the coated particles of tungsten were pressed to form apressed tungsten body the particles were given an additional coating oflead by immersing them in an aqueous solution of lead nitrate in whichthe lead present weighed 30 grams or 3.0% by weight of the tungsten inthe coated particles immersed therein. The particles were well mixed inthe solution which was heated during constant stirring until thesolution was evaporated and the particles dried. These particles werethen screened through a 200 mesh screen and the lead nitrate nowincluded in the coating thereon was reduced to lead by heating thecoated particles in a nickel boat in a reducing atmosphere whereby thelead nitrate was decomposed and reduced. The result was that metalliclead was added to the alloy-like coating on the tungsten particles.These particles were then treated by the same methods described inExample I to form a compressed tungsten body and then a compact of highdensity containing lead in the former voids. The compact produced by themethod described in this example was homogeneous and had a density of15.4 grams per cubic centimeter very close to the full theoreticaldensity of 15.6 grams per cubic centimeter for the density of 10.4 gramsper cubic centimeter (170 grams per cubic inch) of the pressed tungstenbody before the lead was introduced.

Example III In this example 500 grams of tungsten powder having a bulkdensity of substantially grams per cubic inch Were coated with the samemetals and in the same manner as set forth in Exmple I. The entire batchof coated particles was then placed in a mold and again heated andcompressed to a density of 11.6 grams per cubic centimeter (190 gramsper cubic inch). When this density was reached 200 grams of lead wereadded as set forth in Example I, this amount being slightly in excess ofthe theoretical amount required to fill the voids in the pressed body.The actual density of the compact made as described in this example,measured by water displacement, was 16.0 grams per cubic centimeterwhereas the theoretical density, if all the voids were filled, wouldhave been 16.2 grams per cubic centimeter.

Example IV In this example 500 grams of tungsten powder having a bulkdensity of substantially grams per cubic inch were coated with the samemetals and in the same manner as set forth in Example I. The entirebatch of coated particles was then placed in a mold and again heated andcompresesed to a density of 12.2 grams per cubic centimeter (200 gramsper cubic inch). When this density was reached 220 grams of lead wereadded as set forth in Example I, this amount being slightly in excess ofthe theoretical amount required to fill the voids in the pressed body.The actual density of the compact made as described in this example,measured by water displacement, was 16.1 grams per cubic centimeterwhereas the theoretical density, if all the voids were filled, wouldhave been 16.3 grams per cubic centimeter.

Example V One kilogram of pure tungsten powder having a bulk density ofsubstantially 140 grams per cubic inch was first treated with an aqueoussolution containing nickel chloride, cupric chloride and stannicchloride. The amounts of the metal salts present in the batch ofsolution into which the particles were immersed were computed to have acombined metal content of 30 grams or 3.0% of the weight of the tungstenparticla immersed in the solution. This comprised 7.5 grams of tin, 15grams of nickel and 7.5 grams of copper, thus the ratio of the metalcontents of the specific salts used was 50% by weight of nickel and 25%by weight each of copper and tin. With the exception of metal salts andthe total weight of metal present in the metal salt solution, the samesteps were followed, pressures used, temperatures used and Weight oflead added as were followed and used in Example I. The theoreticaldensity of a compact for the density to which the tungsten body waspressed before addition of the lead in this example would be 15.4 gramsper cubic centimeter. The actual density of the compact made asdescribed in this example, measured by water displacement, was 15.3grams per cubic centimeter.

Example VI The composition of matter produced by this example was madein substantially the same manner as that described in Example V exceptthat as described in Example II, before the coated particles of tungstenwere pressed to form a pressed tungsten body the particles were given anadditional coating of lead by immersing them in an aqueous solution oflead nitrate in which the lead present weighed 30 grams or 3.0% byweight of the tungsten in the coated particles immersed therein. Theprocedures, pressures, temperature and weight of lead added from thispoint on were those described in Example I was varied by Example II. Thecompact produced by the method described in this example washomogeneous. The theoretical density of a compact for the density towhich the tungsten body was pressed before addition of the lead in thisexample would be 15.2 grams per cubic centimeter. The actual density ofthe compact made as described in this example, measured by waterdisplacement, was 15.2 grams per cubic centimeter.

What I claim is:

The method of preparing a high density radiation screening compactedmaterial comprising coating tungsten particles with at least one of themetals of the group consisting of lead, tin, copper and nickel,compressing said coated particles between heated rollers thus forming ahoneycomb-like pressed body, further compressing said pressed bodybetween sheets of lead both said body and said sheets of lead beingpassed between rollers heated to a temperature just below that of themelting point of lead causing said lead to flow into the voids in saidpressed body whereby said coated particles are bonded together and thevoids therein are substantially filled with lead forming a compactedbody of high structural strength.

References Cited in the file of this patent UNITED STATES PATENTS;1,342,801 Gebouer Jan. 8, 1920 1,913,133 Stout June 6, 1933 2,034,550Adams Mar. 17, 1936 2,370,242 Hensel et a1 Feb. 27, 1945 2,716,705 ZinnAug. 30, 1955 2,747,105 Fitzgerald May 22, 1956 2,783,145 Boyce Feb. 26,1957 OTHER REFERENCES The Reactor Handbook, vol. 3, p. 8. Published byA.E.C., 1955.

